1. Field of the Invention
The present invention relates to a horizontal type disk injection molding apparatus having mold opening guide means for preventing axial displacement between stationary and movable disk cavity molds when the mold is opened.
More particularly, the present invention relates to a disk injection molding apparatus having mold opening guide means capable of preventing damage to signal bits transferred to the surface of a molded optical disk due to the axial displacement between the stationary and movable disk cavity molds when the mold is opened following completion of the injection molding process.
2. Description of the Related Art
For injection molding of a magneto-optical disk, a disk injection molding apparatus has been used which comprises a stationary mold base, a movable mold base, cylindrical disk cavity molds provided at the center of each of the bases, and a pair of cylindrical guide rings provided concentrically at the outer peripheries of each of the disk cavity molds as alignment guide means. The circumferential surface joint sections of each tip end of each cylindrical guide ring are male and female tapered fitting surfaces.
FIG. 11 is a sectional view of the conventional horizontal type disk injection molding apparatus when the mold is being clamped, and FIG. 12 is a diagram of the conventional apparatus when the mold is being opened.
In these drawings, side A is a stationary mold assembly and side B is a movable mold assembly.
A disk cavity mold 2 is fixed to the stationary mold base 1 of the stationary mold assembly A, and an injection nozzle 3 for injecting a molten resin material is provided at the center thereof. A cylindrical stationary guide ring 4 is integrally fixed to the outer periphery of the stationary mold base 1, and is provided with a male tapered surface 5 at the left tip end surface thereof with an angle of inclination of 3 to 5 degrees. Through a hole in the center of the stationary mold base 1, resin is supplied from a molding nozzle 28 when injection molding is taking place.
A movable disk cavity mold 7 is fixed to a movable mold base 6 of the movable mold assembly B, and a cavity 8, into which a molten resin material is charged, is formed between the stationary disk cavity mold 2 and the movable disk cavity mold 7. A stamper plate 9 is made of pure nickel, and fine signal bits are formed on the surface (right surface in FIG. 11) thereof. The stamper plate 9 is secured to the surface of the movable disk cavity mold 7 with a mounting member (not shown).
A cylindrical movable guide ring 10 is integrally fixed to the outer periphery of the movable mold base 6 or the movable disk cavity mold 7.
A female tapered surface 11 precisely fitting to the male tapered surface 5 of the stationary mold is formed on the right tip end surface of the cylindrical movable guide ring 10.
The molding assemblies A and B as described above are mounted on a stationary platen 12 and a movable platen 13 of the injection mold machine, respectively.
In a state of mold clamping shown in FIG. 11, the molten resin material is charged into the cavity 8 formed between the cavity molds 2 and 7 at a high pressure and molded so as to transfer the fine signal bits of the stamper plate 9 to the resin molded article. Subsequent to the molding, the movable platen 13 is moved backward to open the mold, and the entire resin molded article is removed from the mold.
As shown in FIG. 12, in an optical disk injection molding machine in which the movable platen 13 moves horizontally, the movable platen 13 is held by four horizontal guide rods 14 and driven by a pressure device such as a toggle mechanism and a hydraulic piston (not shown). In this case, an axis &lt;&lt;bb&gt;&gt; of the movable mold assembly B coincides with an axis &lt;&lt;aa&gt;&gt; of the stationary mold assembly A because of the cooperation between the tapered surfaces of the guide rings 4 and 10, during mold clamping. As the mold opening process proceeds a phenomenon occurs whereby the axis of the movable mold assembly B gradually moves downward by the deflection value .beta.according to the downward deflection value .delta. of the guide rods 14 due to a gap formed between a bearing 15 of the movable platen 13 and the guide rods 14, and because of the weights of the movable platen 13 and the movable mold assembly B. Generally, the deflection value .delta. of an injection molding machine having a mold clamping ability of 75 tons or more reaches 50 to 70 .mu.m by actual measurement.
In the case of high speed molding of an optical disk, the mold is opened when the inside of the resin molded article is in a partially molten state and the surface thereof is not completely solidified. Thus, the axial displacement between the stationary mold assembly A and the movable mold assembly B should be maintained at zero until the mold opening distance reaches at least 200 .mu.m. If not, the resin molded article may become deformed or double transferring of the fine signal bits transferred to the resin molded article may occur.
For example, if the angle of incline of the guide ring is 5 degrees, the deflection value .delta. may reach 17.5 .mu.m when the mold opening distance is 200 .mu.m.
Furthermore, during the mold clamping process, the lower portions of each of the tapered guide surfaces are contact loaded for each molding cycle with the total weight of the movable platen 13 and the movable mold assembly B ranging from 300 to 500 kgf as an impact load. Thus, abnormal wearing and a galling of the lower tapered surfaces tends to occur.
In the optical disk injection mold apparatus, avoiding the use of lubricants is required to prevent impurities from getting into cavity 8. To this end, each of the tapered surfaces is subjected to a hardening treatment such as a molybdenum disulfide coating treatment or the like.
In sum, the conventional horizontal type optical disk injection molding apparatus has a number of shortcomings. Deformation of the optical disk, double transferring of the signal bits, and wearing and galling of the fitting surfaces are caused by axial displacement between the molds.